424 Prof. Tyndall on the Absorption and 



surface, or exposed to it, at pleasure, by the action of a move- 

 able screen. The entire space between the pile and the radiating 

 surface could either be rendered a vacuum, offering no resistance 

 to the passage of the calorific rays, or else be filled by a gas the 

 diathermancy of which was to be examined. 



The concurrence of the experiments made with this apparatus 

 and those made with mine is, as I have stated, remarkable. Some 

 differences, however, exist between my friend and myself, a few 

 remarks on which will not be without their use to those who may 

 afterwards enter upon this extensive field of inquiry. 



Experimenting in the ordinary way with his thermo-electric 

 pile — using one of its faces only — Professor Magnus finds that 

 air and oxygen cut off each more than 11 per cent, of the heat 

 emanating from his source, while hydrogen cuts off more than 

 14 per cent.* I, on the contrary, with the most delicate means 

 I could apply, failed to establish the absorption of these gases 

 by experiments made in the ordinary mannerf . In fact it was 

 their neutrality that drove me to devise the principle of com- 

 pensation, briefly referred to at the commencement of this 

 memoir. I was so particular in the experiments which led me 

 to the above negative result, that if the absorption amounted to 

 one-tenth of that found by Professor Magnus I do not think it 

 could have escaped me. Nor do I think that if such an action 

 existed Melloni could have concluded that the absorption of a 

 column of air fifteen times the length of that employed by Pro- 

 fessor Magnus was absolutely insensible. 



In the account of the experiments already published, where 

 my source of heat was also 100° C, I have set down the absorp- 

 tion of air, oxygen, and hydrogen at about 0*33 per cent., which 

 is for air and oxygen thirty times, and for hydrogen over forty 

 times less than that found by Professor Magnus. 



In fixing the above figure for the absorption of these gases, I 

 protected myself by assigning what I knew to be the superior 

 limit of the effect, but I was morally certain at the time that as 

 soon as I could combine sufficient power and delicacy I should 

 make the effect less. This I have done in my present inquiry, 

 and find the absorption of the above gases to be under 01 per 

 cent., which in the case of oxygen is less than T ^o^ n ^ anc ^ m * ne 

 case of hydrogen less than T ^o^ n °f the en?ec ^ obtained by Prof. 

 Magnus with a tube less than half the length of mine. Making 

 every allowance for the difference between our two sources of 

 heat, the discrepancy between us is still enormous. In fact my 

 conclusion is that these gases are practical vacua to radiant heat, 

 and that the mixture of oxygen and nitrogen which constitutes 

 the body of our atmosphere is the same. 



* Page 30. 



t Philosophical Transactions, 1861 j and Phil, Mag, S. 4, vol, xxii. p. 169. 



